Refrigeration circuit with improved liquid/vapour receiver

ABSTRACT

Refrigeration circuit comprising a compressor, a heat-rejecting heat exchanger an expansion valve, a receiver ( 3 ), and a further expansion valve/evaporator to provide cooling. A second heat exchanger ( 24 ) is arranged in an upper gas portion of the receiver and/or a third heat exchanger ( 34 ) is arranged in a lower liquid portion of the receiver. A better liquid/vapour separation of the refrigerant and/or sub-cooling of the liquid refrigerant are achieved.

This invention relates to a refrigeration circuit comprising a firstcompressor device, a heat-rejecting heat exchanger, a first expansiondevice, a receiver having an upper portion and a lower portion, a secondexpansion device, and a first evaporator. The refrigeration circuitfurther comprises a flow path between the upper portion of the receiverand a compressor, the pressure side of which is in flow communicationwith the entrance of the heat-rejecting heat exchanger.

The refrigeration circuit preferably is of the type designed for CO₂ asa refrigerant, but is not limited thereto.

The refrigeration circuit is of the two stage expansion type, whereinthe refrigerant first is expanded in first stage expansion. The firststage expansion provides cooling to complete condensation of therefrigerant in the receiver. Furthermore, the section of therefrigeration circuit extending from the receiver to the compressordevice is at a substantially lower pressure level than the remainingsection of the refrigeration circuit extending from the compressordevice to first expansion device.

It is an object of the invention to provide a refrigeration circuit withan improved receiver.

It is a further object of the invention to provide a refrigerationcircuit with a receiver outputting from its upper portion flash gashaving substantially no liquid droplets therein.

It is a still further object of the invention to provide a refrigerationcircuit with a receiver outputting a sub-cooled liquid refrigerant.

In accordance with one embodiment of the invention there is provided arefrigeration circuit for circulating a refrigerant in a predeterminedflow direction, comprising in flow direction a first compressor device,a heat-rejecting heat exchanger, a first expansion device, a receiverhaving in its interior an upper portion, being in flow communicationwith the first expansion device, and a lower portion, a second expansiondevice being in flow communication with the lower portion of thereceiver, and a first evaporator; and comprising a further flow pathbetween the upper portion of the receiver and the suction side of acompressor, the pressure side of which is in flow communication with theentrance of said heat-rejecting heat exchanger; wherein at least oneelement of the group consisting of the following elements (a) and (b) isprovided: (a) a second heat exchanger is arranged in said upper portionof said receiver, the entrance of the second heat exchanger being inflow communication with the exit of said heat-rejecting heat exchangerand the exit of the second heat exchanger being in flow communicationwith the entrance of said first expansion device, (b) said further flowpath comprises a third expansion device and, downstream thereof, a thirdheat exchanger arranged in said lower portion of said receiver.

The second heat exchanger arranged in the upper portion of the receiverexchanges heat against the vapour contained in the upper portion of thereceiver. Any liquid droplets that may be present in the upper portionof the receiver will be evaporated and entrained into the further flowpath.

The third expansion device and the third heat exchanger arranged inlower portion of the receiver provide sub-cooling the liquid in thelower portion of the receiver. Such sub-cooled liquid refrigerantresults in more efficient cooling effect by the first evaporator andreduces the formation of refrigerant vapour in the section of thecircuit extending from the receiver to the second expansion device.

All in all the improved receiver provides for a more perfect separationinto a gaseous phase of the refrigerant having substantially no contentof liquid droplets, and a liquid phase that is sub-cooled and has lesstendency to vapour formation.

The first compressor device may be a single compressor or a parallelgroup of several compressors. The compressor device may be of the typecomprising a control of its performance, for example by way ofcontrolling its rotational speed dependent on the pressure level of thecompressed gaseous refrigerant to be achieved.

The compressor associated to the further flow path starting from theupper portion of the receiver, may be a further compressor. The suctionside of such further compressor may be at a higher pressure level thanthe suction side of the first-mentioned compressor device, or may be asubstantially the same pressure level as the first-mentioned compressordevice. It is possible to combine the compressor, that is associated tothe further flow path, with the first-mentioned compressor device,either by using one and the same compressor for compressing the gaseousrefrigerant coming from the second expansion device as well as thegaseous refrigerant coming from the upper portion of the receiver, or bycombining the further compressor, that is associated to the further flowpath, into a parallel group of compressors forming the first compressordevice.

In accordance with an embodiment of the invention, the refrigerationcircuit further comprises a branch circuit, branching off from alocation located in a section of said circuit which section extends fromsaid lower portion of said receiver to the entrance of said secondexpansion device; the branch circuit comprising in flow direction afourth expansion device, a second evaporator, and a second compressordevice; and the branch circuit, at its downstream end, being in flowcommunication with the suction side of said first compressor device.

In such embodiment, the branch circuit provides low temperature cooling,for example for deep-freezing purposes. As compared to such lowtemperature cooling, the second expansion device and the firstevaporator provide for medium temperature cooling, for example forkeeping food and beverages at a temperature level of 0 to 10° C.

The refrigeration circuit may comprise one or several second expansiondevices/first evaporators, arranged in parallel, and one or severalfourth expansion devices/second evaporators, arranged in parallel, ifany.

The refrigerant in the refrigeration circuit may be a one-componentrefrigerant or a multiple-components refrigerant.

In the preceding description, reference has been made to variousexpansion devices. It should be stressed that expansion devices ofvarious constructions and designs may be provided. A quite common formof expansion device is an expansion valve. The expansion device may be athrottling device or a throttle valve. The expansion device, dependingon its location, the temperature level, and the pressure level, mayserve to expand liquid refrigerant to gaseous refrigerant or may expandgaseous refrigerant from a higher pressure level to a lower pressurelevel.

This invention further relates to a refrigeration apparatus comprising arefrigeration circuit as disclosed in the present application.

The refrigeration apparatus of this invention may be provided as a heatpump. The technical elements of cooling apparatus and heat pumps are thesame. With the cooling apparatus, the purpose of cooling is the primarypurpose, and the related generation of heat is normally a side effect.With heat pumps, the generation of heat is the desired purpose, whereasthe related cooling effect of the evaporator(s) is normally considered aless useful side effect. This invention also discloses a heat pumphaving a circuit as disclosed in the present application. Such circuitmay be designated a refrigeration circuit because it contains arefrigerant undergoing condensation and evaporation. Some times peopleprefer to use the term working fluid rather than to use the termrefrigerant when describing a heat pump.

A refrigeration circuit containing CO₂ as a refrigerant may be a circuitoperated in transcritical cycle, or may be a circuit operated insubcritical cycle, or may be a circuit operable in transcritical cycleor in subcritical cycle depending on parameters such as environmentaltemperature and pressure level after the compressor device. In typicalapplications such as cooling temperature sensitive products,deep-freezing, cooling buildings, the refrigeration circuit does notreach a subcritical temperature level at the heat-rejecting heatexchanger, at least in summer time season; the circuit is operated intranscritical cycle. In such a situation the heat-rejecting heatexchanger operates as a gas cooler. In case of a subcritical cycle, theheat-rejecting heat exchanger operates as a combined gas cooler andcondenser.

The main functions of the receiver are to permanently keep available asufficient quantity of liquid refrigerant and to provide a separationbetween liquid refrigerant and gaseous refrigerant (vapour). In case oftranscritical cycle, the condensation of refrigerant by means of flashcooling provided by the first expansion device is a further function.

The refrigeration apparatus/heat pump of this invention has a number ofpreferred fields of application. The most important are cooling food andbeverages in shops, restaurants or other locations of storage; coolingother temperature-sensitive products such as pharmaceuticals;deep-freezing; cooling buildings of any sort; cooling cars and any othertype of vehicles in the broad sense, such as aircrafts, ships, railwaycars etc.

This invention further relates to a refrigeration method. In anembodiment of the invention the refrigeration method comprises at leastone step of the group of steps consisting of (i) operating a heat sourcein said upper portion of said receiver, (ii) operating a heat sink insaid lower portion of said receiver.

An exemplary embodiment of the invention will be described in thefollowing. The features of such embodiment are preferred features of therefrigeration circuit of this invention:

FIG. 1 shows a diagram of a refrigeration circuit for elucidating thebasic configuration of such a circuit;

FIG. 2 shows a receiver/separator on a larger scale, which may beincorporated in the refrigeration circuit of FIG. 1.

The total refrigeration circuit shown in FIG. 1 comprises afirst-described (basic) circuit, a second-described further flow path,and a third-described branch circuit, and some additional elements.

The basic circuit, when beginning with a compressor device 6 andprogressing in flow direction of the CO₂-refrigerant, comprises thefollowing elements:

-   -   compressor device 6 or 6 and 6′;    -   conduit 7;    -   heat-rejecting heat exchanger 1 (gas cooler and/or condenser);    -   conduit 2;    -   first expansion valve a;    -   receiver 3;    -   conduit 4;    -   two parallel second expansion valves b and c;    -   two parallel evaporators E2 and E3;    -   conduit 5 back to compressor device 6.

The compressor device 6 comprises three parallel compressors and afurther compressor 6′ to be described in more detail further below. Thesuction sides of the three compressors are supplied by a common supplyspace 20. Typically, the compressor device 6 compresses the suppliedgaseous CO₂ to a pressure in the range of 50 to 120 bar, whereby thetemperature of the gaseous compressed CO₂ is increased to about 50 to150° C. In subcritical operation the pressure of the compressed gaseousCO₂ would typically be in the range of 40 to 70 bar.

The heat-rejecting heat exchanger removes heat from the CO₂. Insubcritical operation, the CO₂ is typically cooled to 10 to 30° C. andcondensed in the heat-rejecting heat exchanger 1; in this case heatexchanger 1 works as a combined gas cooler and condenser. Intranscritical operation, the CO₂ is typically cooled to a temperature of25 to 45° C., without condensation of a substantial part of the CO₂, inthe heat-rejecting heat exchanger; in this case it works as a gascooler. In order to remove heat from the CO₂, the heat exchanger 1 isgas cooled or liquid (water) cooled.

The vapour or liquid/vapour mixture or liquid CO₂ in subcriticaloperation, is expanded by the expansion valve a provided next to thereceiver 3, thereby providing flash gas in an upper portion of thereceiver 3. Typically, the pressure level in the interior of thereceiver 3 is 30 to 40 bar. A lower portion of the receiver 3 containsliquid CO₂. The receiver 3 also acts as a separator of liquid CO₂ andCO₂ vapour.

By the expansion valves b and c the liquid CO₂ is expanded to typicallya temperature of minus 15 to 0° C., resulting in a pressure level oftypically 20 to 35 bar. The evaporators E2 and E3 next to the expansionvalves b and c serve to allow for a complete evaporation of the CO₂ andprovide large cool surfaces, from where the cooling proper originates,typically by air moving by the “cool air is heavier than warm air”principle or moving by forced ventilation.

The compressor device 6 and the receiver 3 are typically mounted in acommon metal frame, also supporting the control equipment of therefrigeration apparatus. The (first) heat exchanger 1, that is aheat-rejecting heat exchanger, normally stands some distance away fromthe compressor device 6 and the receiver 3 and the expansion valve 8,for example outside a building, where it can be cooled best. It isimportant to note that only the section of the basic circuit extendingfrom the pressure side of the compressor device 6 to the exit side ofthe expansion valve 8 is at the high pressure level of typically 50 to120 bar. The remaining section of the basic circuit extending from theexit side of the expansion valve a to the suction side of the compressordevice 6 is at two substantially lower pressure levels, namely typically30 to 40 bar in front of the expansion valves b and c and typically 25to 30 bar in front of the compressor device 6. As a consequence, thesecond-mentioned section of the basic circuit may be designed for suchlower pressure levels, i.e. by using tubes having thinner walls, byusing less sophisticated connections where CO₂ is flowing, and by usingevaporators adapted to the relatively low pressure level.

There is a further flow path, starting at an exit side of the upperportion (vapour portion) of the receiver 3 with a conduit 12 andcontaining an expansion valve e or throttle valve, and finally leadingto the entrance side of the compressor device 6 via a conduit 11. Theexpansion valve e serve to reduce the pressure of the gaseous CO₂ to thelevel existing at the suction side of the compressor device 6.

As an alternative, the expansion valve e may be dispensed with, andthere is just a conduit 12, 15 from the upper portion of the receiver 3to the further compressor 6′. The suction side of such furthercompressor 6′ is at a higher pressure level that the suction side 20 ofthe compressor device 6. The pressure sides of all the compressors 6 and6′ have the same pressure level. Rather than providing the furthercompressor 6′, it is possible to feed from line 15 into one or severalof the compressors of the compressor device 6, but at a stage after afirst compression stage, so that the flash gas is fed into thecompressor device 6 at the right pressure level of the compressors.

Furthermore, FIG. 1 shows a branch circuit comprising the following: Aconduit 8 branches off from the conduit 4 upstream of the expansionvalves b and c; a (fourth) expansion valve d; a second evaporator E4; aconduit 9; a second compressor device 10, and a conduit 11 providingfluid flow connection with the suction side of the first compressordevice 6. The expansion valve d and the second evaporator E4 aredesigned to provide an expansion of the liquid CO₂ to a lower pressurelevel than existing at the suction side 20 of the compressor device 6.The temperature level reached at the evaporator E4 is lower than thetemperature level reached at the evaporators E2 and E3, therebyproviding means for deep-freezing or storing at deep-freezingtemperature. Typical values are 7 to 15 bar and minus 50 to minus 25° C.in the evaporator E4.

Finally, FIG. 1 shows a conduit 13 branching off the conduit 2 (thatleads from the first heat exchanger 1 to the first expansion valve a) toa heat exchanger E1, an expansion valve f being provided in such conduit13. A conduit 14 leads from the heat exchanger E1 to the suction side ofthe further compressor 6′. The heat exchanger E1 exchanges heat againstthe CO₂ flowing through the conduit 2. Since the expansion valve fprovides cool gaseous CO₂, the CO₂ flowing through the conduit 2 iscooled, thereby either assisting in condensation of CO₂ or insub-cooling of liquid CO₂.

FIG. 2 shows a schematically sectional view of the receiver 3 at alarger scale than in FIG. 1. The receiver 3 has in its interior an upperportion 3 a and a lower portion 3 b. A quantity of liquid CO₂ iscontained in the receiver 3, filling the interior of the receiver 3 upto a level 22. Depending on the operational conditions of therefrigeration circuit, the level 22 may be higher or lower than shown inFIG. 2.

The line 2 (providing a fluid flow connection between the exit of theheat exchanger 1 and the expansion valve a, cf. FIG. 1) extends into thereceiver 3 and is connected to a second heat exchanger 24 arranged inthe upper portion 3 a of the receiver 3. There is a further conduit 26,extending outside the receiver 3 and connecting the downstream end ofthe second heat exchanger 24 to the interior of the upper portion 3 a ofthe receiver 3, an expansion valve 28 being provided in such conduit 26.The expansion valve 28 produces flash gas in the upper portion 3 a,which as a consequence is at a lower temperature level than the CO₂flowing through the second heat exchanger 24. Any droplets of liquid CO₂that may be present in the upper portion 3 a, are evaporated. Thisminimizes the potential for erosion of the expansion valve 34 describedin the following paragraph.

The expansion valve 28 has the same function as the expansion valve ashown in FIG. 1. The difference is that the conduit 2 does not leaddirectly to the expansion valve 28, but there is the second heatexchanger 24 upstream of the expansion valve 28. By means of the secondheat exchanger 24, the gaseous CO₂ exiting the upper portion 3 acontains less condensed CO₂ than without the provision of the secondheat exchanger 24.

There is a further conduit 30 leading, outside the receiver 3, from theupper portion 3 a to a third heat exchanger 32 arranged in the lowerportion 3 b of the receiver 3, an expansion valve 34 being provided insuch conduit 30. The downstream end of the third heat exchanger 32 isconnected by a conduit 36 to the suction side 20 of the compressordevice 6. In other words, the expansion valve 34 replaces the expansionvalve e shown in FIG. 1, and the third heat exchanger 32 is provided inaddition.

By passing through the expansion valve 34 the CO₂ becomes cooler, andthe third heat exchanger 32 provides sub-cooling of the liquid CO₂accumulated in the lower portion 3 b of the receiver 3. The liquid,sub-cooled CO₂ exits the lower portion 3 b via conduit 4, as shown inFIG. 1.

The gaseous CO₂ flowing through the third heat exchanger 32 gets acertain overheating which reduces the risk of entrainment of liquid CO₂into the compressor device 6.

1. Refrigeration circuit for circulating a refrigerant in apredetermined flow direction, comprising in flow direction a firstcompressor device, a heat-rejecting heat exchanger, a first expansiondevice, a receiver having in its interior an upper portion, being inflow communication with the first expansion device, and a lower portion,a second expansion device being in flow communication with the lowerportion of the receiver, and a first evaporator; and comprising afurther flow path between the upper portion of the receiver and thesuction side of a compressor, the pressure side of which is in flowcommunication with the entrance of said heat-rejecting heat exchanger;wherein at least one element of the group consisting of the followingelements (a) and (b) is provided: (a) a second heat exchanger isarranged in said upper portion of said receiver, the entrance of thesecond heat exchanger being in flow communication with the exit of saidheat-rejecting heat exchanger and the exit of the second heat exchangerbeing in flow communication with the entrance of said first expansiondevice, (b) said further flow path comprises a third expansion deviceand, downstream thereof, a third heat exchanger arranged in said lowerportion of said receiver.
 2. Refrigeration circuit according to claim 1,wherein said compressor connected to said further flow path is part ofsaid first compressor device.
 3. Refrigeration circuit according to anyone of claims 1 to 2, wherein said refrigerant is CO₂.
 4. Refrigerationcircuit according to any one of claims 1 to 3, wherein said firstcompressor device comprises a parallel group of several compressors. 5.Refrigeration circuit according to any one of claims 1 to 4, furthercomprising a branch circuit, branching off from a location located in asection of said circuit which section extends from said lower portion ofsaid receiver to the entrance of said second expansion device; thebranch circuit comprising in flow direction a fourth expansion device, asecond evaporator, and a second compressor device; and the branchcircuit, at its downstream end, being in flow communication with thesuction side of said first compressor device.
 6. Refrigeration circuitaccording to any one of claims 1 to 5, wherein several parallel firstevaporators are provided.
 7. Refrigeration apparatus comprising arefrigeration circuit as specified in any one of claims 1 to
 6. 8.Refrigeration method, comprising: (a) circulating a refrigerant in arefrigeration circuit comprising, in flow direction, a first compressordevice, a heat-rejecting heat exchanger, a first expansion device, areceiver having in its interior an upper portion, being in flowcommunication with the first expansion device, and a lower portion, asecond expansion device being in flow communication with the lowerportion of the receiver, and a first evaporator; (b) the refrigerationcircuit further comprising a further flow path provided between theupper portion of the receiver and the suction side of a compressor, thepressure side of which is in fluid communication with the entrance ofsaid heat-rejecting heat exchanger; (c) said refrigeration methodcomprising at least one step of the group of steps consisting of (i)operating a heat source in said upper portion of said receiver, (ii)operating a heat sink in said lower portion of said receiver. 9.Refrigeration method according to claim 8, wherein the refrigerant isCO₂.